The present invention relates to a method for operating a mobile radio communications system, as well as to a subscriber station and a base station which are suitable for implementing such method.
In a mobile radio communications system, such as the UMTS system, several antennas are employed on a base station for the purpose of exploiting spatial diversity. For transmitting data on dedicated channels to individual base stations, the antennas are respectively weighted with different weighting factors. Such weighting factors are generally complex numbers consisting of an amount component and a phase component. A set of weighting factors employed simultaneously for a dedicated channel is also referred to by the term weighting vector. As a result of weighting, a radiation beam in the direction of the location of the respective mobile radio station is produced for each subscriber station in a cell (beam forming).
To allow correct decoding of the useful data symbols transmitted on a dedicated channel, a subscriber station requires a channel estimation indicating at least the phase shift occurring on the channel. Such channel estimation is conventionally performed by transmitting pilot bits on the dedicated channel which are known to the subscriber station so that it can determine the channel's phase shift by comparing received symbol phases of the pilot bits with expected phases.
A first preferred application of the present invention is estimating the dedicated channel of the downlink transmission from a base station to a subscriber station in a mobile radio communications system with antenna diversity.
Channel estimation via the dedicated channel's pilot bits (also referred to below as dedicated pilot bits) is conventionally of poor quality and very noisy. This occurs because, like the rest of the channel, the dedicated pilot bits are transmitted at a power level selected as low as possible in order to minimize interference on other subscriber stations for which the dedicated channel is not intended.
A method of channel estimation is also known, particularly in the case of CDMA systems, whereby global pilot signals, which like the dedicated pilot bits are pre-defined signals, are transmitted from the base station. Being transmitted unweighted, they can be received throughout the mobile radio cell provisioned by the base station (or within the provisioned cell sector in the case of a sectorized base station). Since global pilot signals contain useful information for all the mobile stations within the cell's domain, there is the possibility of the global pilot signals interfering with the dedicated channels. The global pilot signals are therefore transmitted at a higher level of power than the dedicated pilot bits and permit a higher quality estimation. It must be added, however, that an estimation obtained by the subscriber station for a global channel is not automatically transferable to a dedicated channel because the relative phases with which a global and dedicated channel are transmitted from the individual antennas of the base station differ depending on the weighting vector employed. If the subscriber station is aware of the weighting vector employed, it can calculate an estimation of the dedicated channel from an estimation of the global channel. However, this knowledge is only present in the case of a system employing what is termed closed-loop beam forming, where the subscriber station is aware of the weighting vector to be used by the base station having selected it itself and having specified it to the base station. The subscriber station will, however, lack this knowledge in that case if it is prevented by an interruption in transmission from receiving the base station's global pilot bits or from transmitting the selected weighting vector, or if it has not yet had an opportunity to specify a weighting vector to the base station when closed-loop beam forming is being initialized. Calculation is generally not possible in the case of open-loop systems where the base station selects the weighting vector.
Another preferred application of the present invention is controlling beam forming in a mobile radio communications system whereby, from a multiplicity of long-term weighting vectors, a base station selects or linearly combines a short-term weighting vector employed for downlink beam forming with the aid of short-term feedback information supplied by the subscriber station. A system of this type is described, for example, in German patent application DE 100 32 426.6 submitted by the applicant on Jul. 4, 2000.
A provision of conventional systems of this type is that the long-term weighting vectors are respectively determined and transmitted to the base stations by the subscriber stations. The transmission capacity available for transmitting both the short-term feedback information and long-term weighting vectors is limited to a total of one bit per timeslot of the uplink signal. The short-term feedback information has to be transmitted frequently to allow beam forming to adapt quickly to fast fading; this, however, is blocked or will respond more sluggishly if new long-term weighting vectors have to be transmitted to the base station.
Each individual subscriber station within the system must, in a system of this type, also be able to determine the long-term weighting vectors, which increases the complexity of the subscriber stations.